The point of the last two lines was to contrast the sublime efforts of the heavenly choir, the “angels” who “delight to hymn thee above” with that of ourselves: “frail children of dust”, the “humbler creation” with “feeble lays” (inadequately expressed sentiments), who can only “lisp” i.e. sing defectively, imperfectly. I can’t help but feel the punch has been pulled for a lesser purpose.